Combine harvesters are used in agriculture for harvesting crops which are gathered from the field, threshed-out, and separated. The crops obtained by threshing and separating are subsequently freed from undesired crop residues in a cleaning. The cleaning comprises a blower, which acts on back and forth-moving sieves with an air flow, wherein lighter particles, such as chaff, are carried forth by the air flow and the grain falls through the sieves and is taken to a grain tank. As a rule, an upper sieve is placed above a lower sieve.
The usually used, so-called plate sieves are composed, for examine, like a sieve 8 shown in FIG. 1 and representing the state of the art (see, for example, EP 1 068 793 A1). The sieve 8 includes a rectangular frame 10 with plates 12, articulated, in a swiveling manner, around their longitudinal axles. The plates 12 are connected, in a fixed manner, with crankshafts 14, which engage in a slot 16 in an adjusting rod 18, which can move in a longitudinal direction of the sieve 8 with a manual or externally operated drive, so as to adjust the angle of the plates 12 and thus the opening size of the sieve 8—that is, the distance between the lower side of the back part of a plate 12 and the upper side of the back part of the downstream plate 12 and, if possible, to be able to adapt optimally to the individual harvesting conditions, such as grain size, lateral slope, and throughput. The plates 12 are cut in an undulating manner on their back ends and are also curved in an undulating manner in the transverse direction of the sieve 8. As can be seen in FIG. 2, which shows a side view of a conventional adjusting rod 18, and in FIG. 3, which shows a longitudinal section through a conventional sieve 8, the plates 12 of a sieve 8 have been arranged at an equal distance up to now, as is the case with the slot 16 in the adjusting rod 18. However, since identical plates 12 were used over the entire rod 8, the opening sizes d of the sieve 8 are constant over the length of the sieve.
During operation, the upper sieves are loaded on their front end or in its immediate vicinity with a mixture of grain and undesired crop residues, which reach the sieve via a falling stage which is impinged on with an air flow directed toward the back, whereas the lower sieves are loaded from the upper sieve and, accordingly, also contain more grain in the front area and more impurities in the back area. With both sieves, the air flow leads to a separation between the grain and impurities, so that most of the grain reaches the sieve further in front than the lighter impurities, carried back by the air flow. In this way, separation curves are typically produced in accordance with FIG. 4, in which the solid curve (a) shows the separation of grain and the broken curve (b), the separation of impurities over the length of the sieve 8. One can see that most of the grain is separated in the front half of the sieve, whereas the amount of the separated grain toward the back decreases successively, which is caused by the different striking points of the grain and the impurities on the sieve after the air flow has passed. In the back area of the sieve, the separation of the grain from the impurities is more difficult than in the front area, since whereas in the falling stage and in the front sieve area, a separation takes place according to pneumatic coefficients of the components, there is a greater importance in the back area of the separation according to geometric parameters, so that the adjustment is more sensitive there and the sieve opening has to be adjusted there in a relatively precise manner and must be brought to a rather narrow magnitude, adapted to the grain size, so as to attain an optimal result of the cleaning. Since the sieve opening is adjusted constant and relatively small over the length of the sieve, there is no sufficient separation of the grain then in the front area of the sieve. A greater fraction of the grain accordingly moves back via the sieve and is not sieved out, but rather arrives as lost grain on the field or in the returns conveyor.
In order to avoid the accumulation of excess material quantities in the front area of the sieve, U.S. Pat. No. 7,896,731 B2 proposes the provision of some plates with sections that are wider in the lateral direction and deeper in the longitudinal direction than the other plates in the front area of an upper sieve. All plates of the sieve, however, are at equal intervals from one another and have the same angular positions. In this way, the sieve openings with the aforementioned plates are somewhat larger and a greater air flow penetrates there, from the bottom, through the sieve, so that the separation of grains will be improved, on the one hand, on the aforementioned plates and the impurities will be blown away to a greater extent, on the other hand.
U.S. Pat. No. 7,997,967 B2 describes a sieve in which affixing sections of the plates can be introduced into one of several slots of the sieve frame, so as to produce various angles of the plates and to adapt the sieve to various types of crops. By the selection of various slots, it is also possible to set up various angles for different sections of the sieve.
EP 1 49 351 A1 proposes that the upper sieve and the lower sieve be composed of front and back sections, whose opening size can be changed by means of a common drive. The drive can be uncoupled from the back section, so that only the front section is adjusted and the back section remains in the individual position. This solution refers, above all, to the back part of the upper sieve. This is often further adjusted so as to be able to separate ears which have not yet been threshed to the returns and send them back to the threshing process.
Finally, U.S. Pat. No. 7,566,268 B1 provides plates with only a few fingers, in the middle of the sieve, which are curved in the shape of an arc, are longer and are at a greater distance from one another in the longitudinal and transverse direction than the fingers of the other plates and are meant to be used to bring the corncobs into a longitudinal orientation, in which they penetrate the sieve openings and can remain in them thus forming obstacles for stems and husks moving over the sieve.
The sieves according to U.S. Pat. Nos. 7,896,731 B2 and 7,566,266 B1 have the disadvantage that different types of plates are needed for a single sieve, which complicates the maintaining of a supply inventory and the ordering of spare parts. In the arrangement according to EP 1 849 351 A1, the adjustment of the part of the sieve that is uncoupled from the drive proves to be cumbersome and in the embodiment according to U.S. Pat. No. 7,997,967 B2, relatively large gaps arise between adjacent plates, through which the undesired high quantities of impurities can penetrate, if the front plate is to be set flatter than the plates which follow toward the back.